1. Field of Invention
The present invention relates to a signal transmission structure, circuit board, and connector assembly structure. More particularly, it relates to a signal transmission structure, circuit board, and a connector assembly structure with increased impedance continuity on a signal transmission path.
2. Description of Related Art
For large-scale printed circuit board and packaging substrate having electrical connections for signal lines between two elements or terminals, the wire width in the signal line has to be consistent. Thus, during electronic signal transmission, the characteristic impedance of the signal wire can remain the same. Especially during high speed and high frequency signal transmission, a superior impedance matching design between the two terminals is required for reducing reflection due to impedance mismatch, thus lowering the signal transmission insertion loss. And at the same time, the return loss for signal transmission is increased to prevent signal degradation and to improve transmission quality.
FIG. 1A is a top view for a conventional circuit board with an upright Sub-Miniature Type A (SMA) connector, FIG. 1B and FIG. 1C are the cross-sectional views through line A-A′ and line B-B′ of FIG. 1A respectively; whereas, FIG. 1D and FIG. 1E are the respectively top views of FIG. 1B at the conductive layer and the reference plane. Referring to FIG. 1A through FIG. 1E, a conventional signal transmission structure 100 disposed on a circuit board 20 includes a conductive layer 110 and a reference plane 120. But the conductive layer 110 and the reference plane 120 are parts of components which form the circuit board 20. The conductive layer 110 is disposed on the surface layer of the circuit board 20. In other words, the conductive layer 110 is the uppermost conductive layer of the circuit board 20. The conductive layer 110 includes a signal perforated pad 112, a first line segment 114, a plurality of first alignment perforated pad 116, and a reference plane 118. The first line segment 114 is connected to the signal perforated pad 112. Whereas, the first alignment perforated pad 116 is formed from several portion of the reference plane 118. In addition, the reference plane 118 and the reference plane 120 include a ground plane or a power plane. The reference plane 120 contains a plurality of second alignment perforated pads 122 separately corresponding to the first alignment perforated pads 116.
According to the above descriptions, the upright SMA connector 50 is a coaxial cable connector, and having a signal pin 52 with several alignment pins 54. Wherein, the signal pin 52 is inserted into the corresponding signal through hole 22 of the circuit board 20, passing through the signal perforated pad 112, and coming into physical contact with the signal perforated pad 112, thus allowing for signal transmission through the signal perforated pad 112. Furthermore, the alignment pins 54 are inserted into the corresponding alignment through holes 24 of the circuit board 20, securely disposing the SMA connector 50 on the circuit board 20. In addition, the alignment pins 54 are inserted through the corresponding first alignment perforated pad 116 and the second alignment perforated pad 122 for electrically connecting to the reference plane 120. As a result, a plating through hole (PTH) 24a is formed at the alignment through hole 24 for reinforcing the integrated mechanical strength of the alignment pin 54 and the circuit board 20. In addition, the reference plane 120 includes an insulating pin 124 for preventing short circuit during contact with the signal pin 52.
In the conventional signal transmission structure 100, the characteristic impedance of the signal perforated pad 112 is less than that of the first line segment 114; and the difference of characteristic impedance between the two is dramatic, which leads to increasing of insertion loss during signal transmission, thus affecting signal transmission quality especially during high frequency transmission.